So We Live Forever Taking Leave.
We can only speculate that it would have been about Lowry's time at the University of Cambridge as tripos relates to the University examination system. The University of Cambridge divides the different kinds of honours bachelor's degree by tripos (listen). The word has an obscure etymology, but may be traced to the three-legged stool candidates once used to sit on when taking oral examinations. An apocryphal legend says that students used to receive one leg of a stool in each of their three years of exams, receiving the whole stool at graduation. Another tradition holds that the name is because of the three brackets printed on the back of the voucher.
Lowry described the eight papers in his tripos to Conrad Aiken in a letter dated 24th April 1931. (Collected Letters Pg 95)